1. Field of the Invention
This invention relates to a frequency mixer circuit using field effect transistors (FETs).
2. Description of Related Art
FIG. 1 shows a circuit provided with a conventional frequency mixer circuit using FETs. The circuit shown in FIG. 1 comprises a frequency mixer circuit using a dual gate metal oxide semiconductor (MOS) FET Q2, a radio frequency (RF) amplifier using a MOS FET Q1 and a local oscillator using a bipolar transistor Q3 which are disposed at the prior stage to the frequency mixer circuit. An RF signal VRF inputted from an antenna is amplified through the RF amplifier and sent to the frequency mixer circuit to be mixed with a signal VLO supplied from the local oscillator thereby to output a signal thus obtained as an IF output signal IFOUT. In FIG. 1, L1 to L4 are coils, T1 is an output transformer, C1 to C15 are capacitors, CV1 to CV3 are variable capacitors, C1A, C1B and C1C are ganged variable capacitors, R1 to R10 are resistors, and VSS is a source voltage.
In the conventional frequency mixer circuit shown in FIG. 1, the output signal VRF from the RF amplifier is applied to one gate of the MOS FET Q2 and the output signal VLO from the local oscillator is applied to the other gate of the MOS FET Q2. The one gate of the MOS FET Q2, which is provided with the output signal VRF, is also applied through the resistor R5 with a voltage obtained by dividing the source voltage (-VSS) through the resistors R6 and R7. This is for the so that the gate voltage of the MOS FET Q2 is fixedly set by an optimized direct current bias voltage thereby to control the conversion gain of the FET Q2.
With such a conventional frequency mixer circuit, the gates of one FET Q2 are respectively supplied with the output signals VRF and VLO for mixing these frequencies and, as a result, a local oscillator signal is required which is so high in voltage level (as high as several volts) that a square characteristic can be found in the drain current of the FET Q2. For this, an amplifier having a large output is required to amplify the local oscillator signal as well as the circuit current becomes large.